Nanopowders have unique physical properties that are directly related to their small size and high specific surface area. Nanopowders exhibit an inherent propensity to agglomerate, resulting in an increase of their apparent particle size. Agglomeration has a direct impact on the functional properties of the nanopowder such as their optical and magnetic characteristics as well as the catalytic and conductive properties.
Because of their high specific surface area, nanopowders are very reactive and difficult to handle. The deposition of a thin film, or other coating material on the outer surface of the individual particles, prevents their agglomeration and provides for their safe handling without compromising their unique properties.
The choice of coating material, i.e. polymer-type or other, provides for a selective control over the surface characteristics of the powder. The hydrophilicity of a powder can be modified, in addition to controlling other intrinsic properties, by surface treatment of the powder and/or by the proper selection of a coating material. A stable pyrophoric nano-aluminum powder (ignites readily at ambient temperature) can be created by the application of a thin polymeric film coating the surface of the particles. Such a coating provides for a stable powder at lower temperatures while not adversely affecting its high energetic value at higher temperatures.
Plasma surface treatment has been previously used as a surface modification technique to enhance the hydrophobicity, hydrophilicity, adhesion, and corrosion resistance of a great many substrates, including polymeric films. It has also found widespread use in cleaning and etching applications.
Plasma deposition and plasma polymerization techniques have been developed to apply thin coatings, e.g. polymeric films, onto a variety of substrates. Most of these techniques operate at fairly low pressures (smaller than 100 Pa).
Thin film-coating has been previously reported as changing the surface properties of nanopowders, while decreasing their agglomeration and improving their dispersion characteristics. The coating of zirconia (ZrO2) nanopowders (˜130 nm) with a polyethylene film, using an RF plasma torch (27 MHz) operating at low pressure (30 Pa), has been reported by He et al (1).
The coating of alumina (Al2O3) nanoparticles (˜10-150 nm) with a polypyrrole film, using an RF plasma torch (13.56 MHz) operating at low pressure (25 Pa), has been reported by Shi et al. (2). A thin polypyrrole film was deposited at a discharge power of 10 W. A fluidized bed kept under vacuum was used to introduce the alumina nanopowder (0.16 g/min). Shi et al. also reported on the deposition of a polystyrene film on nanocarbon tubes using a similar process (3).
The coating of alumina (Al2O3) nanoparticles with an ethane-based polymeric layer having a thickness of about 1.5 nm, using an RF plasma torch (13.56 MHz) operating at low pressure (1 kPa), has been reported by Schallehn et al. (4). Coated alumina (Al2O3) nanoparticles were produced at a rate of 0.5-1 g/h and at yields of about 40%.
A microwave (MW) plasma torch operating at high frequency (2.45 GHz) and low pressure (1-5 kPa) has been reported by Vollath et al. to coat nano-oxide powders such as zirconia (ZrO2), alumina (Al2O3), tungsten oxide (WO2, WO3), hafnium oxide (HfO2), tin oxide (SnO, SnO2), and iron oxide (Fe2O3) (5, 6). The film coating was achieved using methyl methacrylate as the polymer precursor. The monomer was introduced at the exit of the plasma torch discharge and was polymerized under the influence of the UV radiation emitted from the plasma.
The preparation and coating of silver nanoparticles with a polymeric layer, using a MW plasma torch operating at high frequency (2.45 GHz) and low pressure has been reported by Lik Hang Chau et al. (7). The same author also reported on the preparation and coating of cobalt nanoparticles with a silicon carbide layer, using a MW plasma torch (8). CoCl2 and SiCl4/Hexane were the precursors for the preparation and coating respectively.
The coating of fine silica powders ranging in size from 30-80 nm, using a capacitive plasma torch (13.6 MHz) operating at low pressure (1-5 kPa), was described by Kouprine et al. (9). The plasma discharge power was set at 700-1500 W and the plasma gas was comprised of a mixture of argon and, methane or ethane. A fluidised bed was used to introduce the silica powder feed material.
The synthesis and carbon-coating of iron nanoparticles by means of laser pyrolysis, using a continuous wave CO2 laser operating at a power setting of 120 W, a wavelength (λ) of 10.6 micrometers and a pressure of 700 mbar, has been reported by Dumitrache et al. (10). Iron carbonyl and acetylene were the precursors for the powder synthesis and coating respectively.
The synthesis and carbon coating of aluminum particles using a DC plasma arc discharge torch (1-50 V; 30-150 A) operating at atmospheric pressure has been reported by Ermoline et al. (11). The cathode was reported as being composed of copper, while the anode was comprised of a consumable aluminum rod. Ablation of the anode was carried out in pulse mode to produce coated nano-aluminum particles. The carbon coating was achieved using natural gas.
The coating of porous granulated silica particles (˜150 μm) with a thin film of plasma-polymerized tetrafluoroethylene (TFE), using an Atmospheric Pressure Glow Discharge (APGD) in a specially designed plasma discharge torch (15 kHz; 100 kPa; 10 W), has been reported by Sawada et al. (12). The plasma feed gas was comprised of helium and TFE (1%). The silica particles were reported as being recirculated several times through the plasma region.
The carbon coating of copper nanoparticles using a DBD torch operating at atmospheric pressure was reported by Lei et al. (13). Copper nanoparticles were produced using a flow levitation method wherein a copper wire is heated with high frequency electromagnetic coils. The copper nanoparticles produced were subsequently carbon coated in situ by means of a DBD torch using argon, hydrogen and methane and operating at atmospheric pressure.
Bretagnol et al. (19) studied the surface modification of low density polyethylene (LDPE) powder in a low pressure RF plasma operating at 13.56 MHz and using nitrogen and ammonia as the processing gas. The powder was recirculated in a fluidized bed reactor. Residence times in the order of 300 seconds were needed to alter the particles' wettability.
Polyethylene powders have also been treated as disclosed by Leroy et al. (20). The plasma discharge was coupled to a fluidized bed reactor and the powder was treated in the after glow region of the plasma. The processing gas was a mixture of oxygen and nitrogen. A microwave plasma having a frequency of 2450 MHz was used and operated at low pressures of 0.1 to 20 mbar.
The present disclosure refers to a number of documents, the content of which is herein incorporated by reference in their entirety.